The present invention relates generally to pulse oximetry systems and specifically to a method and apparatus for determining whether an oximetry sensor is properly in contact with a patient.
Pulse oximeters measure and display various blood flow characteristics including but not limited to the oxygen saturation of hemoglobin in arterial blood. Oximeters transmit light through blood perfused tissue such as a finger or an ear, and photoelectrically sense the absorption of light in the tissue. The light passed through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of a blood constituent present in the blood. The amount of light absorbed is then used to calculate the concentration of the blood constituent.
A wide variety of oximeter sensors are available from a number of manufacturers which provide the interface between the patient and the oximeter. Oximeter sensors typically include two LEDs of different wavelengths which alternately transmit light into the tissue, and a detector for receiving the light after passing through the tissue. In processing the detector signal received from the sensor, it is important that the oximeter have accurate information regarding the state of the sensor. That is, the oximeter must be able to reliably determine whether the sensor is on or off the patient. If the oximeter receives inaccurate information regarding the state of the sensor, it may issue false alarms regarding the patient's health, or more seriously, may fail to report an emergency condition when one exists.
Previous generation oximeters attempt to detect the SENSOR OFF condition. However the reliability of these detection schemes is not well documented. One oximetry system addresses the problem by providing for a timeout period during which either no pulses or pulses unacceptable for oximetry calculations are detected. If a timeout occurs, it is assumed that the sensor may not be in contact with the patient, and the oximetry system ceases to display saturation and pulse rate. However, recent oximetry algorithms are more robust in that they are able to use or selectively ignore previously unusable data, e.g., data corrupted by motion artifact, during the determination of blood constituent concentrations instead of reporting a pulse timeout. As a result, the pulse timeout algorithm is no longer suitable for determining whether the sensor remains in contact with the patient.
In addition, the pulse qualification of one previous embodiment relied on a decision tree implemented using ad hoc and non-uniform logic. The thresholds or coefficients of such algorithms were typically selected manually resulting in a sub-optimal separation of the regions of interest in the input space. Moreover, such decision trees do not typically represent non-linear relationships.
It is therefore desirable to provide a reliable method for determining whether a pulse oximeter sensor is in proper contact with a patient.